High affinity, stereospecific recognition sites (receptors) for neurotransmitters, neuromodulators, and many clinically useful drugs have been identified in both peripheral tissues and the central nervous system. The interaction of a neurotransmitter, neuromodulator or drug with a specific recognition site initiates a series of events (for example, the opening of an ion channel or activation of an enzyme) resulting in either a physiological/behavioral response (in the case of a neurotransmitter or neuromodulator) or a pharmacological effect (in the case of a drug), Furthermore, such observations suggest that endogenous substances may also be present that mimic (or antagonize) the effects of exogenously applied substances. Studies are in progress to characterize "recognition-effector" systems, and to link novel recognition sites to effector systems in both peripheral tissues and the central nervous system in order to define the physiological roles of these systems. "Recognition-effector" systems under study include: a) the benzodiazepine/GABA receptor chloride ionophore complex; b) the glycine-gated chloride ionophore; c) "peripheral-type benzodiazepine receptors (in both peripheral tissues and the central nervous system); d) receptors for central stimulants (e.g. amphetamine, methylphenidate); e) recognition sites for hallucinogens (phencyclidine), and f) recognition sites for compounds that regulate voltage-sensitive calcium channels.